Heart valve prostheses have been used successfully since 1960 and generally result in improvement in the longevity and symptomatology of patients with valvular heart disease. However, 10-year mortality rates still range from 30-55%. Currently available biological-based heart valves, due to calcification and degradation, generally require replacement in the 8 to 15 year time frame whereas mechanical heart valves, which last longer, require chronic drug regimens to prevent blood clotting. A dilemma still exists on what type of valve to implant into patients between the ages of 50 and 65 years old. Polymer trileaflet valves offer natural valve hemodynamics with the potential for sufficient durability for long-term use. Unfortunately, these valves have not been successful to date because of long-term material degradation in vivo through a combination of oxidative reactions with blood and the high dynamic tensile and bending stresses borne by the material. This proposal describes a suitable material that is enjoying unheard of success as coatings on coronary stents and, with the appropriate valve design, stands a very good chance of functioning as a totally synthetic polymer heart vane prosthesis. The goal of the Phase I SBIR project was to select the appropriate materials and designs for the heart leaflet valve and to demonstrate that the new valve will have a high likelihood of displaying sufficient biocompatibility along with improved fluid dynamics and long-term durability. These goals were successfully met. The Specific Aims and goals of the Phase II proposal are as follows: 1. Comparative accelerated wear testing on a minimum of three valves at three tissue annulus diameters for the equivalent of 15 years of performance (6xl0/8 cycles) on a Quatromer composite valve and a commercially available tissue valve. The valves will be periodically examined for wear, material fatigue, and hemodynamic performance. Finally, failure modes after further cycling will be determined. 2. Comparative chronic in vivo performance of the Quatromer composite and commercially available tissue valve will be assessed in an ovine (sheep) animal model. A minimum of eight animals (six with Quatromer composite valves, two with tissue valves) will survive an implantation period of at least 20 weeks. Hemodynamic and laboratory measurements will be performed periodically followed by explanation, valve evaluation, and histopathology. The completion of Phase II will provide the necessary feasibility to transfer the project to Phase III, commercialization of the valve. The primary outcome of Phase II will be the data required for an application to the FDA to obtain a Feasibility Investigational Device Exemption (IDE). Clinical trials will follow this approval.